Halliday & Matthiessen (2014: 190):
The propositional (ideational) type occur only with declarative clauses. They appear at the same locations in the clause as the mood Adjuncts — though for different reasons: they are less integrated into the mood structure, being located rather according to their significance for the textual organisation of the clause. In particular, they are strongly associated with the boundary between information units — realised as a boundary between tone groups: hence the commas that typically accompany them in writing. So they often occur medially, following the item which is prominent; otherwise, they may occur as Theme, frequently as a separate information unit, or in final position as Afterthought.